Algae are mainly water plants dwelling on ponds, rivers, seas, lakes, oceans, ditches and other water bodies. Some species are also found on stones, the bark of trees, fences, etc generally in moist environment.
They are photosynthetic organisms that have chlorophyll and can make their own food. Fungi do not have chlorophyll so these organisms cannot make their own food as algae do. These are decomposers that survive either consuming dead matter or eating off other organisms.
The green algae have an important bearing upon our lives both for good and for evil. In course of their food-making activity, they add oxygen to the water thus making more of this essential gas available for fish